1. Field of the Invention
The present invention relates to nonaqueous electrolyte secondary batteries using a negative-electrode active material composed of powdered silicon and/or a silicon alloy and nonaqueous electrolytic solutions for nonaqueous electrolyte secondary batteries.
2. Description of Related Art
Portable electronic equipments, electric power storage systems, and the like have employed, as power sources, nonaqueous electrolyte secondary batteries that use a nonaqueous electrolytic solution through which lithium ions move between a positive electrode and a negative electrode for charging and discharging. Such nonaqueous electrolyte secondary batteries widely use a graphite material as a negative-electrode active material in the negative electrode.
Meanwhile, mobile equipments such as mobile phones, notebook personal computers, and PDAs have been increasingly smaller and lighter in recent years, and multifunctionalization of such equipments requires higher power consumption. Thus, nonaqueous electrolyte secondary batteries that are used as power sources for such equipments have been required to have much lighter weight and much higher capacity.
However, a negative-electrode active material using the graphite material does not have sufficient capacity in the graphite material, and therefore cannot satisfy such requirements.
On this account, it has been studied to use a material capable of forming an alloy with lithium, such as silicon, germanium, and tin, for a negative-electrode active material having high capacity. In particular, silicon has a high theoretical capacity of about 4000 mAh per 1 g. Thus, it has been studied to use silicon or a silicon alloy for the negative-electrode active material.
However, the use of a material capable of forming an alloy with lithium, such as silicon, for the negative-electrode active material leads to a larger volume change associated with absorption and desorption of lithium in comparison with the use of the graphite material for the negative-electrode active material. Hence, a battery using such material has a problem of deterioration due to expansion and contraction by charging and discharging. Furthermore, a material capable of forming an alloy with lithium, such as silicon, is readily reacted with a commonly used nonaqueous electrolytic solution. Thus, the reaction with the nonaqueous electrolytic solution degrades a negative-electrode active material such as silicon to reduce charge and discharge cycle characteristics.
In the present invention, as described later, the nonaqueous electrolytic solution contains additives including at least one fluorinated lithium phosphate selected from the group consisting of lithium monofluorophosphate, lithium difluorophosphate, and lithium trifluorophosphate and a diisocyanate compound.
Japanese Patent Laid-open No. 2008-277004 discloses a nonaqueous electrolyte secondary battery using an electrolytic solution containing lithium difluorophosphate. However, there is no discussion of the effect in the nonaqueous electrolyte secondary battery when it uses a negative-electrode active material composed of powdered silicon and/or a silicon alloy.
Japanese Patent Laid-open No. 2007-242411 discloses a nonaqueous electrolyte secondary battery using an electrolytic solution containing a diisocyanate compound. However, there is no disclosure of the effect when a negative-electrode active material composed of powdered silicon and/or a silicon alloy is used and of the effect when the fluorinated lithium phosphate described above is used in combination.